Comparative analysis of molten salt electrolytes for solid carbon production

The excessive amount of carbon dioxide in the atmosphere is a global problem, and thus, the mitigation of carbon dioxide emissions should be an urgent issue. Technically, CO₂ can be converted into valuable solid carbon through electrolysis in a molten salt electrolyte. Although studies have been conducted on the conversion of CO₂ into various morphologies of solid carbon, the molten salt itself has received less attention in the research literature.

Based on the inorganic mixture composition, various molten salts for CO₂ electrolysis are classified into four categories: molten chloride, molten chloride-oxide, molten chloride-carbonate, and molten carbonate. This review aims to fill the gap in the literature by describing the state of the art of each molten salt category with some experimental data collected in previous studies. The exploration is followed by an investigation of the performance, focusing on the efficiency, specific electrical energy consumption, corrosion of electrodes, and the presence of metal elements in the carbon product. The use of molten carbonate eliminates the potential of corrosion from Cl₂ and has a reported Faraday efficiency of up to 94.9%. However, electrode corrosion occurs in all types of molten salt, leading to the presence of metal elements in the solid carbon product. The economic perspective on this CO₂ electrolysis in molten carbonate is also discussed as an important area that should be attended to when considering future potential commercialization. Finally, this review provides guidance on selecting molten salt electrolyte by taking account their advantages and disadvantages.